Editing Devon (section)

===Geology===
{{See also|Geology of Dartmoor National Park|Geology of Exmoor National Park}}
[[File:Geologic map Wales & SW England EN.svg|thumb|Geological map of Wales & Southwest England]]

A geological dividing line cuts across Devon roughly along the line of the [[Bristol to Exeter line]] and the [[M5 motorway]] east of Tiverton and Exeter. It is a part of the [[Tees–Exe line]] broadly dividing Britain into a southeastern lowland zone typified by gently dipping [[sedimentary rock]]s and a northwestern upland zone typified by [[igneous rock]]s and folded sedimentary and [[metamorphic rock]]s.

The principal geological components of Devon are i) the [[Devonian]] [[stratum|strata]] of north Devon and south west Devon (and extending into Cornwall); ii) the [[Culm Measures]] (north western Devon also extending into north Cornwall); and iii) the granite [[intrusion]] of Dartmoor in central Devon, part of the [[Cornubian batholith]] forming the 'spine' of the southwestern peninsula. There are [[clastic rock|blocks]] of Silurian and Ordovician rocks within Devonian strata on the south Devon coast but otherwise no pre-Devonian rocks on the Devon mainland. The metamorphic rocks of Eddystone are of presumed Precambrian age.<ref>Edmonds, E. A., et al. (1975) ''South-West England''; based on previous editions by H. Dewey ([[British Geological Survey]] UK Regional Geology Guide series no. 17, 4th ed.) London: HMSO {{ISBN|0-11-880713-7}}</ref>

The oldest rocks which can be dated are those of the Devonian period which are approximately 395–359&nbsp;million years old. Sandstones and shales were deposited in North and South Devon beneath tropical seas. In shallower waters, limestone beds were laid down in the area now near Torquay and Plymouth.<ref>{{cite book |title=Devon's Geology: An Introduction |last=Hesketh |first=Robert |publisher=Bossiney Books |year=2006 |isbn=978-1-899-383-89-4}}</ref>
This geological period was named after Devon by [[Roderick Murchison]] and [[Adam Sedgwick]] in the 1840s and is the only British county whose name is used worldwide as the basis for a geological time period.<ref>{{cite web |title=Devon Geology Guide – Devonian Slates, Sandstones and Volcanics |last=Laming |first=Deryck |author2=Roche, David |url=http://www.devon.gov.uk/geo-devonianslatessandstonesandvolcanics.pdf |access-date=14 May 2014 |url-status=dead |archive-url=https://web.archive.org/web/20131110021248/http://www.devon.gov.uk/geo-devonianslatessandstonesandvolcanics.pdf |archive-date=10 November 2013}}</ref>

Devon's second major rock system<ref>{{cite web |url=http://www.devon.gov.uk/index/environmentplanning/natural_environment/geology/geology-guide.htm |title=Devon's Rocks – A Geological Guide |publisher=Devon County Council |access-date=18 May 2011 |url-status=dead |archive-url=https://web.archive.org/web/20160303212321/http://www.devon.gov.uk/index/environmentplanning/natural_environment/geology/geology-guide.htm |archive-date=3 March 2016}}</ref> is the Culm Measures, a geological formation of the [[Carboniferous]] period that occurs principally in Devon and Cornwall. The measures are so called either from the occasional presence of a soft, sooty coal, which is known in Devon as ''culm'', or from the contortions commonly found in the beds.<ref>{{cite book |last=Edmonds |first=E. A. |author2=McKeown, M. C. |author3=Williams, M. |others=Dewey, H. |title=South-West England |publisher=HMSO/British Geological Survey |location=London |year=1975 |edition=4th |series=British Geology |page=34 |chapter=Carboniferous Rocks |isbn=0-11-880713-7}}</ref> This formation stretches from Bideford to [[Bude]] in Cornwall, and contributes to a gentler, greener, more rounded landscape. It is also found on the western, north and eastern borders of Dartmoor.

The sedimentary rocks in more eastern parts of the county include [[Permian]] and [[Triassic]] sandstones (giving rise to east Devon's well known fertile red soils); [[Bunter (geology)|Bunter pebble beds]] around Budleigh Salterton and Woodbury Common and [[Jurassic]] rocks in the easternmost parts of Devon. Smaller outcrops of younger rocks also exist, such as Cretaceous [[chalk]] cliffs at Beer Head and gravels on Haldon, plus [[Eocene]] and [[Oligocene]] [[ball clay]] and [[lignite]] deposits in the Bovey Basin, formed around 50 million years ago under tropical forest conditions.